This invention relates to the focusing of an optical print head upon a discontinuous print medium secured to a print drum by a clamp and, more particularly, to the use of coaxially disposed focusing and write laser beams to enable close positioning of an objective lens to the print medium during writing, and including lens-positioning apparatus for locating the lens at a reference position for passage over the clamp during rotation of the drum.
Optical printers employing a photosensitive recording medium and a laser beam for writing alphanumeric and pictorial data on the recording medium are in wide use today, these printers including laser printers for outputting hard copy from computers and facsimile machines. The printers employ various optical systems for focusing and directing a laser beam upon the recording medium. The printers employ various transports for moving the recording medium past a print station for illumination by the laser beam, and various electro-optical devices for controlling the intensity of the laser beam and for modulating the laser beam. It is of particular interest to increase the speed of the writing, and also to increase the resolution of an image produced on the print medium so as to provide a higher output printing rate and improved image quality.
While various forms of photosensitive materials may be employed four the print medium, it is advantageous to employ a photosensitive material having a binary characteristic in terms of its response to incident radiation such that, below a threshold intensity of incident radiation, there is no imprinting of marks on the material. For radiation incident upon the material with an intensity above the threshold, marks are imprinted upon the material. The binary characteristic may include also a frequency response in that, by way of example, a minimum threshold value of frequency is required to activate the photosensitive material and that, for radiation of a lower frequency, there is no imprinting of marks. Such a binary characteristic enhances the production of half-tone images formed by an array of loosely spaced dots, as well as gray-scale images wherein the density of dots is varied within each pixel to produce a gray scale.
A problem arises in that there is a need for higher resolution images than is available with present printing systems. Also, it is desirable to include numerous shades of gray by the distribution of dots of varying density within the pixels of the image. However, currently available printers are limited in their capacity for performing high-resolution gray-scale images, and are also limited in the speed at which such images can be produced.